linux-kernel - Re: [PATCH] loop: Make explicit loop device destruction lazy

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Date:	Sat, 29 Sep 2012 07:50:26 +0200
From:	Jens Axboe <axboe@...nel.dk>
To:	Jeff Moyer <jmoyer@...hat.com>
CC:	Dave Chinner <david@...morbit.com>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] loop: Make explicit loop device destruction lazy

On 2012-09-28 17:02, Jeff Moyer wrote:
> Jens Axboe <axboe@...nel.dk> writes:
> 
>> On 2012-09-28 08:09, Dave Chinner wrote:
>>> From: Dave Chinner <dchinner@...hat.com>
>>>
>>> xfstests has always had random failures of tests due to loop devices
>>> failing to be torn down and hence leaving filesytems that cannot be
>>> unmounted. This causes test runs to immediately stop.
>>>
>>> Over the past 6 or 7 years we've added hacks like explicit unmount
>>> -d commands for loop mounts, losetup -d after unmount -d fails, etc,
>>> but still the problems persist.  Recently, the frequency of loop
>>> related failures increased again to the point that xfstests 259 will
>>> reliably fail with a stray loop device that was not torn down.
>>>
>>> That is despite the fact the test is above as simple as it gets -
>>> loop 5 or 6 times running mkfs.xfs with different paramters:
>>>
>>>         lofile=$(losetup -f)
>>>         losetup $lofile "$testfile"
>>>         "$MKFS_XFS_PROG" -b size=512 $lofile >/dev/null || echo "mkfs failed!"
>>>         sync
>>>         losetup -d $lofile
>>>
>>> And losteup -d $lofile is failing with EBUSY on 1-3 of these loops
>>> every time the test is run.
>>>
>>> Turns out that blkid is running simultaneously with losetup -d, and
>>> so it sees an elevated reference count and returns EBUSY.  But why
>>> is blkid running? It's obvious, isn't it? udev has decided to try
>>> and find out what is on the block device as a result of a creation
>>> notification. And it is racing with mkfs, so might still be scanning
>>> the device when mkfs finishes and we try to tear it down.
>>>
>>> So, make losetup -d force autoremove behaviour. That is, when the
>>> last reference goes away, tear down the device. xfstests wants it
>>> *gone*, not causing random teardown failures when we know that all
>>> the operations the tests have specifically run on the device have
>>> completed and are no longer referencing the loop device.
>>
>> I hear that %^#@#! blkid behavior, it is such a pain in the neck. I
>> don't know how many times I've had to explain that behaviour to people
>> who run write testing with tracing, wonder wtf there are reads in the
>> trace.
>>
>> Patch looks fine, seems like the sane thing to do (lazy-remove on last
>> drop) for this case.
> 
> Do we also want to prevent further opens?

That's not a bad idea, at least it would be the logical thing to do. But
it does get into the realm of potentially breaking existing behaviour.

-- 
Jens Axboe

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